A 7-year-old boy who fatally shot his cousin with an illegal, unregistered gun Sunday night is in police custody along with his mother, police told WKTV-Boston. The pair are being question by police for the shooting of Laquarrie Jefferson.

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"This is a case with two victims — you have a victim who was shot and you have a 7-year-old who has to live with the fact that this happened for the rest of his life," police commissioner Davis told reporters. "So it is hard to call anybody a suspect in this case, except for that person who brought that gun into the house."

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Denise Watts said LaQuarrie's relatives are struggling to come to terms with the shooting.

"I don't have a clue where he got this gun, but just I hope that this is a wake-up call to all the young mothers in Boston: Guns and babies do not mix," Watts said.

At the John P. Holland Elementary School in Dorchester, where LaQuarrie had just finished first grade, teachers burst into tears when they heard the news Monday, said Principal Michele O'Connell.

"When he'd see you, he'd want to give you a hug," said Jo-Ann Penn, director of instruction at the school. "If he knew he did something wrong, he would want to apologize. He was just innocent."

Police Commissioner Edward Davis said the boy was shot once in the abdomen and was pronounced dead several hours later at Boston Medical Center.

Davis said the boy's mother was in the apartment, in a four-story brick building on Seaver Street across the street from Franklin Park, when he was shot. He added that there were people who lived in the boy's apartment who were "related to gangs."

Reporters could not reach members of the family for comment after police restricted access to the building to residents. LaQuarrie's mother could not be reached by telephone.

The Associated Press contributed to this report.